A typical automotive transmission utilizes a common input shaft providing drive power from an engine to a series of gears (which also may be referred to as “pinions”) located on one or more drive shafts (which also may be referred to as “layshafts”) that are aligned and intermeshed with corresponding gears located on a parallel output shaft (which also may be referred to as a “countershaft”). Specific combinations of corresponding pinions on the layshaft and gears on the countershaft may be selected to provide different gear ratios for transmitting torque at different, typically overlapping, rotational speed ranges.
More specifically, the combinations of corresponding pinions and gears provide gear ratios for transmitting torque and rotation from the engine to downstream components of a drive train, such as a differential system, and ultimately to wheels operably connected to the drive train. When the pinion of the layshaft drives the corresponding gear of the countershaft for rotation, the resultant gear ratio is commonly referred to by a certain gear number. For example, first gear of the transmission system can be the result of the gear ratio between a “first gear” pinion on the layshaft and an intermeshed and corresponding “first gear” gear on the countershaft. The term gear ratio is used broadly herein to encompass any reduction in torque or speed between two rotating elements. For example, the term gear ratio encompasses ratios between gears having intermeshed teeth, and ratios between sprockets rotated via a common chain.
For certain transmissions, a pair of co-axially arranged drive or layshafts is provided. Each of the layshafts selectively transmits torque to a series of pinions, typically alternating between the layshafts as the diameter of the pinions increases. The pinions located on the layshaft intermesh with the corresponding gears located on the countershaft to provide the desired gear ratios. The countershaft, in turn, transmits the torque and rotational speed to the downstream components of the drive train at the selected gear ratio. In this manner, for example, an odd layshaft transmits torque and rotation to pinions and gears that may be selected to provide first, third, and fifth gear ratios, and an even layshaft transmits torque and rotation to pinions and gears that may be selected to provide second, fourth, and sixth gear ratios. Hence, the pinions on the layshafts and gears on the output or countershaft are often referred to by their position in the relative progression of gear ratios provided by the transmission (i.e., first gear pinion and first gear second gear pinion and second gear, etc.).
Transmissions having co-axially arranged layshafts are often referred to as dual-clutch transmissions when each of the layshafts is equipped with an independently-engageable clutch system for selectively engaging and disengaging the layshafts to supply torque and rotation through selected gear ratios to the countershaft. The desired combination of pinions and gears to provide the desired “gear,” i.e., gear ratio, is selected through an automatic or manual shifting system. The clutch systems may be selectively engaged to transfer torque and rotation from the engine input shaft, through the selected clutch system and layshaft, and through the selected combination of corresponding pinions and gears to provide torque and rotational speed to the countershaft, and then to other components of the drive train, determined by the input shaft torque and speed and the selected gear ratio.
Upon selection of an intermeshed pinion and gear combination, the rotational speed of the pinion and/or gear typically must be synchronized with the rotation of its respective layshaft or countershaft. To facilitate this adjustment, for example, the countershaft often includes multiple synchronizers that can be individually activated, such as by using hydraulics, to engage a desired gear of the countershaft in driving rotation to gradually bring the gear, pinion and/or layshaft to the same rotational speed as the countershaft. The gear and countershaft are then locked to prevent further relative rotation. Thus, when a particular gear ratio is selected, the rotational speed or the countershaft is matched to the speed of the selected gear of the countershaft which is driven for rotation by the corresponding pinion located on the selected layshaft.
The resultant torque and speed of the output shaft or countershaft is determined by the torque and speed of the common input shaft, modified in the transmission by the gear ratio between the selected pinion and gear combination. For example, to select first gear, the odd layshaft clutch may be engaged to transmit torque from the input shaft to the odd layshaft. Rotation of the odd layshaft causes each of the pinions mounted thereon, such as first gear pinion, third gear pinion and fifth gear pinion, to rotate.
Rotation of the odd layshaft causes the first gear pinion, third gear pinion, and fifth gear pinion to drive each of the corresponding gears of the countershaft for rotation therewith. However, each of the corresponding gears of the countershaft is allowed to free-wheel relative to the countershaft unless engaged. When first gear is selected, the synchronizer located on the countershaft and associated with first gear is activated to engage the first gear with the countershaft for rotation therewith. The first gear pinion mounted on and driven by the odd layshaft is intermeshed with the first gear on the countershaft, and thus drives the countershaft through the first gear rotation, which in turn rotates the drive train. A similar procedure is followed for the other gears, and this sequence can be reversed to shift from a higher gear (such as second gear) to a lower gear (such as first gear).
The transmission is typically located in an engine compartment which also includes many other components, such as the engine, the radiator and coolant system, battery, etc. In most modern vehicles, the engine compartment affords little spare room or space. As a result, the design of a vehicle requires consideration of the size and geometry of each component to be installed within the engine compartment. Oftentimes, a pre-determined size and geometry of one or more of the components results in, or dictates, a particular availability of other components due to space restrictions within the engine compartment. Furthermore, the design of the engine compartment and the vehicle body are often influenced by minimal packaging requirements for the operational components. In some instances, a savings of 10% of the size and/or weight of the transmission is considered significant from a commercial standpoint.
In many dual clutch transmissions, the synchronizers are located on the countershaft, due to the arrangement of the co-axial layshafts and countershaft and the desire to minimize the length of the transmissions. For example, this arrangement can permit the pinions on the layshafts to be more closely spaced than if the synchronizers were located on the layshafts and between the pinions. However, such a transmission arrangement has disadvantages.
For example, each of the synchronizers must have a capacity sufficient to transfer torque between the countershaft and the selected gear located on the countershaft. The required torque capacity of the synchronizers is a function of the square of the differential speed (w) between the countershaft and the selected gear prior to their engagement and the rotational inertia of the gear, which in turn is a function of the diameter of the gear and other factors. The diameter of the gears can vary depending upon the desired gear ratios. For example, in first gear it often is desirable to provide to countershaft with a reduction in rotational speed and an increased torque relative to the torque and speed of the input shaft. In order to accomplish this, the first gear pinions often are of a small diameter, which is limited by the diameter of its respective layshaft, and the corresponding first gear on the countershaft is of very large diameter. The second gear pinion located on the layshaft typical is of a larger diameter than the first gear pinion, and the second gear located on the countershaft is of a smaller diameter than the second gear, and so on.
Due to the comparatively large reduction in speed and increase in torque desired for the first gear ratio, the synchronizer torque capacity for the first gear of the countershaft often must be significantly greater than the synchronizer capacities for the other gears. Because the synchronizer for first gear is mounted on the countershaft, the synchronizer must have sufficient torque capacity to compensate the additional torque load imposed by the relatively high gear ratio and rotational inertia for first gear, which may include reflected inertia from the pinion and layshaft. In a typical five speed transmission, for example, the gear ratios may be as follows: 4.12 (first gear); 2.17 (second gear); 1.52 (third gear); 1.04 (fourth gear); 0.78 (fifth gear); and 3.32 (reverse gear).
In general, the more capacity that is required of the synchronizer, the larger and more costly the synchronizer is. Therefore, in order to minimize the costs of such transmissions, a variety of different synchronizers having different capacities are used. For example, the synchronizers for first gear typically are larger and more costly than the synchronizers for the other gears, and may be of a different more complex construction, such as multi-cone synchronizers, raising additional durability and service issues.
The arrangement of the pinions and gears, in addition, limits the minimum diameters (perpendicular to the axes of the layshafts) and the minimum lengths (parallel to the axes of the layshafts) of the transmission. For example, the diameter of the first gear, typically the largest gear diameter, is often a factor that limits efforts to reduce the diameter of the overall transmission. The number of gears on the countershaft, in addition, can be a limiting factor on the minimum length of the transmission, as the gears are typically aligned in series, with a separate countershaft gear provided for each of the gears of the layshafts for the different gear ratios.
Accordingly, it is desired to provide components providing flexibility in the design, installation, and selection of transmission components installed within the engine compartment. In particular, it is desired to provide an vehicle transmission that can be configured to a selected geometry, is reduced in cost and can operate with a reduced complexity of design constrictions.